Until now, a method for forming a multilayer coating film by using a 3-coat 2-bake (3C2B) process has been widely used as a method for forming a coating film on automobile bodies. This method sequentially comprises applying an electrodeposition coating composition to a substrate, and performing heat-curing, followed by application of an intermediate coating composition, curing by baking, application of a base coating composition, preheating (preliminary heating), application of a clear coating composition, and curing by baking.
However, in recent years, for the purpose of saving energy, consideration has been given to omitting the bake-curing step that is performed after application of an intermediate coating composition, and to using a 3-coat 1-bake (3C1B) process sequentially comprising the following steps: application of an intermediate coating composition, preheating (preliminary heating), application of a base coating composition, preheating (preliminary heating), application of a clear coating composition, and curing by baking. From the viewpoint of reducing environmental pollution caused by the vaporization of organic solvents, the establishment of a 3-coat 1-bake process using aqueous coating compositions as the intermediate coating composition and the base coating composition is particularly desired.
However, in the 3-coat 1-bake process described above, which uses an aqueous intermediate coating composition and an aqueous base coating composition, a resulting coating film is likely to have insufficient smoothness and distinctness of image due to the formation of a mixed layer from the aqueous intermediate coating composition and the aqueous base coating composition.
In order to solve these problems, Patent Literature (PTL) 1 discloses a method for forming a multilayer coating film, the method comprising (1) providing a target substrate on which an electrodeposition coating film is formed; (2) applying an aqueous intermediate coating composition to the electrodeposition coating film to form an intermediate coating film; (3) sequentially applying an aqueous base coating composition and a clear coating composition to the intermediate coating film by a wet-on-wet process without curing the intermediate coating film to form a base coating film and a clear coating film; and (4) simultaneously curing by baking the intermediate coating film, base coating film, and clear coating film. In relation to such a method, Patent Literature 1 discloses that when the aqueous intermediate coating composition contains a specific acrylic resin emulsion and urethane resin emulsion, and also when the intermediate coating film formed from the aqueous intermediate coating composition has a specific water absorption rate and water elution rate, the formation of a mixed layer from the intermediate coating composition and base coating composition is effectively prevented, and that a multilayer coating film having excellent surface smoothness can thereby be formed.
Patent Literature (PTL) 2 discloses a method for forming a coating film, the method comprising a step of sequentially applying an intermediate coating composition, a base coating composition, and a clear coating composition to an electrodeposition-coated material; and a step of simultaneously curing by baking the three layers formed in the above step. The base coating composition of PTL 2 has a feature in that it has a viscosity at 60° C. (V is (60° C.)) of 10 to 1,000 poise, a viscosity at 90° C. (V is (90° C.)) of 1 to 500 poise, and a viscosity at 120° C. (V is (120° C.)) of 100 to 10,000 poise, and a V is (60° C.)/V is (90° C.) of 1.5 to 10 and a V is (120° C.)/V is (90° C.) of 2 to 20, when the nonvolatile content of the base coating composition is 90 wt %. PTL 2 further discloses that the nonaqueous dispersion contained in this base coating composition functions as a viscosity-imparting agent and prevents blur or inversion from occurring between the layers, and that a coating film excellent in appearance with high distinctness of image and gloss can thus be formed.